Uplink (UL) Multiple-Input Multiple-Output (MIMO) was introduced in Long Term Evolution (LTE) Release 10 for up to 4 transmit (Tx) antennas at the user equipment (UE). MIMO can be used to improve the UL data transmission performance by UL beamforming and spatial multiplexing and to improve the UL control-channel performance by transmit diversity. An UL precoder is determined by a base station (BS) and is conveyed to the UE as part of a scheduling grant. Typically, the BS determines the precoder by first estimating an UL channel matrix by measuring received UL sounding reference signals (SRS) transmitted by the UE. The BS can then evaluate predicted performance for a set of pre-defined candidate precoding matrices specified by a so called codebook and select the one which gives the highest performance. The aim of precoding is to achieve the best possible data reception at the receiver. The BS then only needs to signal, to the UE, an index of the codebook entry that it has selected, referred to as precoder matrix indicator (PMI). The UE should then use the precoder matrix determined by the BS in its data transmission.
LTE uses Orthogonal frequency-division multiplexing (OFDM)-based waveforms for the transmission. To enable high power amplifier efficiency on the UE side it is important to use a transmission with low cubic metric. The cubic metric is a metric of the reduction in power capability of a typical power amplifier. Therefore, Discrete Fourier Transform (DFT)-spread OFDM is used in the LTE UL data transmission. In order to preserve the good cubic metric properties of DFT-spread OFDM when UL precoding is used, all precoder matrices map the layers to the antenna ports such that at most one layer is mapped to each antenna port; i.e. each antenna transmits a single-carrier waveform and thereby preserves the cubic metric of DFT-spread OFDM.
In new radio (NR) access currently being specified for the next generation of wireless communication, 5G, it is foreseen that the number of UE transmission (Tx) antennas will increase. Systems operating at much higher carrier frequencies than today are expected, e.g. operating in the millimeter wave (mmW) bands. The main motivation for going up in frequency is the availability of spectrum in these bands. However, this also poses challenges in radio network design due to the high propagation loss associated with high frequencies. This excess propagation loss can be mitigated by using beamforming at the BS and/or the UE. Since the physical size of an isotropic antenna decreases with increased frequency, there is an opportunity to accommodate a larger number of antennas within a given device form factor. This enables high gain beamforming also on the UE side. Different UE antenna architectures for high frequencies are currently being discussed. One option is to use one- or two-dimensional arrays of elements having wide angular coverage. Another option is to have several directive antennas covering different angular sectors.
Overall and prevailing challenges and requirements in these considerations are that the UEs should have good coverage in order to keep user satisfaction, and as a means to meet this desire, any interference needs to be controlled.